Electrical power system for steel rolling mills



y 3, 1956 M. BANTRIM 2,752,803

ELECTRICAL POWER SYSTEM! FOR STEEL ROLLING MILLS Filed April 10, 1951 2 Sheets-Sheet 1 INVENTOR M. B. ANTRlM July 3, 1956 ELECTRICAL POWER SYSTEM FOR STEEL ROLLING MILLS 2 Sheets-Shes t 2 Filed April 10, 1951 Z 000- v. mumun=2 PZwKMDO 0 & 6 4 2 O 2 4 6 8 v INVENTOR ATTgRNEvs 30 TIME SECONDS United States Patent ELECTRICAL POWER SYSTEM FOR STEEL ROLLING MILLS Malcolm B. Antrim, Coatesville, Pa., assignor to Lukens Steel Company, Coatesville, Pa., a corporation of Pennsylvania Application April 10, 1991, Serial No. 220,310

Claims. (Cl. 80-31.1)

This invention relates to electric power drive systems, and is particularly concerned with a power supply system for feeding one or more driving motors which are subject to rapid and heavy fluctuations in load.

The invention is of especial advantage and utility in the reversible type rolling mill, for instance in large mills for the rolling of steel, such for example as plate mills, blooming mills, slabbing mills, etc. For convenience of description the invention is herein disclosed as applied to a plate mill.

In a plate mill of the kind in question a pair of rolls are arranged to be reversibly driven so that a plate, or an ingot or slab from which a plate is to be made, may be repeatedly passed in opposite directions between the rolls of the mill, the rolls being progressively brought closer together between passes so as to effect the desired reduction, and thus form a plate of the desired gauge.

This type of rolling operation is characterized by unusually large and rapid fluctuations in load, the most severe of which result from the entrance of the plate between the rolls and the exit thereof at the opposite side, as the plate is passed back and forth. These rapid and extensive fluctuations are communicated to the driving system for the rolls. Other rapid and heavy fluctuations in load are also present, including those incident to deceleration and acceleration of the drive system, both of which occur each time the roll drive is reversed.

For many reasons which need not be considered herein, it is advantageous to employ electric drive motors for the rolls of a mill of the kind referred to. Most advantageously these motors are direct current motors driven by direct current generators associated therewith. Because of the power requirements of a large rolling mill, the electric power is desirably supplied by the use of a plurality of direct current generators, connected in parallel. It is with this type of power system, incorporating a plurality of parallel generators, that the present invention is concerned.

The operation of the generators in installations of this kind has presented very severe problems arising in large part from the difliculty of maintaining proper operation of the commutation mechanism of the generators at the times when the rapid and large fluctuations in load produce transients in the electrical circuits. The difliculties in question have heretofore resulted in very serious arcing and sparking, leading to flashover between various elements of the electrical system or to ground. At times this has occurred with such severity as to approach an explosive condition which is extremely dangerous to personnel. Extensive damage to equipment is also likely to result. Moreover, these commutation difficulties have represented an exceedingly severe maintenance problem, requiring frequent grinding of the commutation and replacement of brushes and other parts.

Usually the foregoing difficulties tend to occur more pronouncedly with respect to one of the generators of the group used than with respect to the others.

Patented July 3, 1956 I have found that these highly undesirable conditions arises during the load fluctuations, with their attendant transients, even where special care is taken to utilize generators having essentially identical internal characteristics. The problem has not even been met by attempts to accurately match the magnetic characteristics of the stators and rotors of the generators.

Indeed, I have found that even the use of generators having interpole and compensating windings and otherwise constructed to minimize armature reaction and to equalize their electrical characteristics, has not solved the difficulty.

While various of the above expedients are more or less effective in most types of electrical power systems, especially where the load remains substantially constant during normal operation, nevertheless even these special precautions do not overcome the commutator problems present in the type of equipment here involved where exceedingly heavy and rapid fluctuations in load represent the usual rather than the unusual condition of operation. In fact, with all of these prior arrangements flashover is likely to occur in a driving system for a reversible plate rolling mill, and usually a greater tendency will be present for this effect to take place with respect to one generator as compared with the other or others of the group.

I have discovered, however, that the tendency toward sparking, arcing and flashover can be greatly diminished by the adoption of a specially arranged system of connections or buses between the paired or grouped generators and the motor or motors used for driving the rolls. As will further appear, the bus system of the present invention substantially equalizes the transient loads as between generators of the group, during times of rapid load fluctuation; and in consequence of these improve ments the danger to personnel and equipment is greatly diminished and the maintenance problems are enormously reduced.

How the foregoing objects and advantages are attained will appear more fully from the following description referring to the accompanying drawings, in which- Figure 1 is a somewhat diagrammatic isometric view of a rolling mill installation, including generators and motors, this view also indicating in a diagrammatic manner certain of the electrical interconnections arranged according to the invention;

Figure 2 is a current-time curve for one of the roll driving motors of Figure 1;

Figure 3 is a schematic diagram of various of the electrical connections of the invention; and

Figure 4 is another diagram of certain other connections.

Referring first to Figure 1, a pair of mill rolls are indicated at 4 and 5, these rolls having necks 6 and 7 and being adapted to be adjusted with respect to each other so as to vary the distance therebetween for rolling of a plate such as indicated at P.

In the installation illustrated each roll is driven by a reversible direct current motor, such as shown in outline at 8 and 9. Shafts 10 and 11, driven by the motors, are respectively connected with the roll necks 6 and 7 by means of universal joints 12 and 13 and intermediate shafts 14 and 15, this system of shaft connections providing freedom for adjustment of the rolls toward and away from each other.

In the particular installation illustrated, current is supplied to the motors 8 and 9 by four generators 16, 17, 18 and 19. The armatures of all four generators are driven by a motor 20 and this system may also include a flywheel 21.

In an installation of the kind referred to just above adapted for the rolling of heavy steel plate, the power requirements of the mill may run even into thousands of kilowatts. indeed, in a large mill to which this invention has been applied, the peak power requirements run as high as about 18,000 kw. With the direct current motors for driving the rolls adapted to operate at a suitable voltage, say for example 600 volts, these power requirements correspond to a maximum current running up to as high as 30,000 amperes or even somewhat higher.

From the above it will be understood that the commutation problem, even at best, is a very difficult one to handle. This is especially true because of the rapid and large fluctuations in load which occur during each cycle of operation of the mill, i. e., each time the plate is passed back and forth between the rolls.

The magnitude of these fluctuations is graphically shown in Figure 2, which illustrates the current drawn by one of a pair of roll driving motors in an installation of the kind illustrated in Figure 1. The current is plotted against time in seconds. Figure 2 should be read from right to left. At the extreme right of the figure the current curve is at a level corresponding to idling of the roll. As the plate enters the rolls, the current rapidly rises to a positive peak R. For a brief interval the current remains high while the plate is passing between the rolls. The current then rapidly drops to a negative peak r. This drop and reversal in current represents the exit of the ingot at the opposite side of the rolls and also the reversal of motor drive, the peak 1' representing the roll reversing current peak. Beyond the peak r the current again diminishes until the ingot reenters the rolls at the opposite side whereupon the current rapidly rises to the negative peak R. When the ingot again leaves the rolls the current rapidly drops again to the zero value and then rises to a peak 1' in consequence of reversing the motor and mill roll and then accelerating them in the other direction. After the roll is up to speed the current drops again to a low positive value until the reentrance of the plate being formed between the rolls, and this again causes a sharp rise to a peak R Once again the plate leaves the rolls and the current drops to zero and then rises again to a negative peak r representing reversal of the motor and roll and acceleration of the roll. After the roll has been brought up to speed the current again diminishes until the plate again re-enters, whereupon the negative peak R results. This process, is of course, continued for each pass of the plate between the rolls, as is clearly shown in the graph of Figure 2.

in further explanation of the graph of Figure 2 it is pointed out that as the plate is rolled, it becomes longer and therefore the heavy rolling current at each of the ma or peaks (R, R, R R etc.) is maintained for a longer mterval of time, as is plainly shown in the graph. Thus, the peak R is of substantially greater extent on the time scale than are the earlier peaks, for instance R From the description of Figure 2 above it will be seen that the fluctuations of load occur with great rapidity and are further of very large magnitude. Indeed, in many nstances it will be noted that exceedingly large changes In current occur almost instantaneously, many of these changes being as large as 5,000 to 10,000 amperes. Since two motors are used in the installation of Figure 1 and since the load peaks of both motors substantially coincide, the total change in load on the generators will be substantially twice that just indicated. It is this exceedingly high rate of change which represents a special problem in an installation of the kind here under consideration; and it is to this problem that the present invention is directed, as will now be shown.

For reasons which need not be considered herein, the power requirements for the roll driving motors are met by the employment of a multiplicity of generators. While for mills of certain size a pair of only two generators may be needed, in a, large mill, such as diagrammatically illustrated in Figure 1, it is preferred to employ four such generators arranged in the manner to be described. I

It should also be kept in mind that generators of a variety of types may be utilized, although it is especially advantageous to use generators of the compound wound type each having, in addition to the armature and its windings, a shunt field winding, a series-differential field winding and a series-cumulative field winding. Moreover, these generators desirably have interpole and compensating windings of the kind used for neutralization of armature reaction.

Whatever the number of generators employed (for instance, two, three, four, etc.), the series field windings are desirably cross connected. This provides equal load division under steady state load conditions. Thus, in the case of an installation incorporating four generators, as herein illustrated, cross connections are used between all four of the generators for this purpose. This is shown in Figure 3 and described just below.

Figure 3 indicates diagrammatically the two motors 8 and 9, and the four generators similarly appear at 16, 17, 18 and 19. From the standpoint of certain of the connections generators 16 and 17 are coupled as a pair, and generators 18 and 19 are coupled as another pair. Thus, the series-cumulative windings c and c of generators 16 and 1'7 are interconnected by a bus 22. A bus 23 v interconnects the corresponding windings of the generators of the other pair. The two bus connections 22 and 23 are interconnected by an intermediate bus bar 24, and this latter bus bar is connected with the main bus 25 which serves to feed the motors.

The series-cumulative windings c of generator 17 is connected with the series-differential winding d of generator 16, as is clearly shown; and the series-cumulative winding 0 of generator 18 is similarly connected with the series-difierential winding d of generator 19. The c winding of generator 16 is connected with the d winding of generator 18, and correspondingly, the c winding of generator 19 is connected with the d winding of generator 17.

The armatures of the four generators are indicated at a, a a and a and armatures are connected in series with the series-difiierential windings and the interpole and compensating windings 1', i i and i as is shown in the figure. The other sides of the armatures are interconnecting in pairs, a bus 26 serving to interconnect the armatures of generators 16 and 17 and a bus 27 similarly interconnecting the armatures of generators 18 and 19. An intermediate bus 28 interconnects the armatures of buses 26 and 27 and the bus 28 is in turn connected with the main bus 29 adapted to feed the motors.

The motors 8 and 9 are also coupled in parallel, buses 30 and 31 serving this purpose; and the main buses 25 and 29 deliver respectively to the buses 30 and 31. For an installation of this kind, it is preferred that the two motors have essentially the same electrical characteristics.

Circuit breakers such as indicated at 32 are advantageously included in the circuit between each motor and each generator.

Reversal of the motors is efiected by reversal of the be seen these exciting windings are connected in parallel and fed from a separately excited generator E. The exciting winding E of this generator is fed from an external source of power through the reversing switch indicated at S, By this system the current in the shunt windings,

s, s s and s may be reversed and this in turn reverses the current fed to the motors from the generators.

Some of the connections, notably the external bus bar connections are also shown in Figure l with the same identifying reference characters.

Further features of the arrangement and the manner of operation thereof are brought out more fully herebelow.

The generators should have substantially identical electrical characteristics; and as has been noted above, in electrical power installations subject only to constant load, the coupling of essentially similar generators (having suitable electrical characteristics) in parallel provides a system which will function without appreciable commutation difiiculty. However, even when employing generators whose series fields are cross connected and even where the generators are carefully designed from the standpoint of neutralization of armature reaction, pronounced commutator difliculties tend to arise under the transient conditions, i. e., the conditions of high timerate-of-change of current. In a reversible plate mill these conditions of time-rate-of-change of current are exceptionally severe, but notwithstanding the severe conditions I have found that with paralleled generators, and with the special external bus bar system utilized in this invention, the commutator difiiculties are enormously reduced.

In the bus bar system of the invention provision is made for establishing substantially equal time constants in all of the complete circuits through any motor and any generator, for example, as illustrated in Figure 3 the time constant of the complete circuit, including any motor, the bus connections to any generator, through the cross connections to the associated generator and from the latter through the bus connections to the other side of the motor, is the same as any other similar circuit in the system. For this purpose, in the arrangement as shown in Figure 3 the intermediate bus bar 24 is connected with the buses 22 and 23 at points dividing the buses 22 and 23 into portions of substantially equal resistance. Similarly, the main bus 25 is connected with the intermediate bus 24 at a point dividing the intermediate bus into portions of equal resistance. The same general arrangement is utilized with respect to intermediate buses 28 and its point of connection with the armature buses 26 and 27; and also with the connection of the main bus 29 to the intermediate bus 28. An essentially similar pattern is followed with reference to the main buses 25 and 29 with the motor buses 30 and 31.

With the bus bar system just described and with cross connections between the series field windings of the generators of equal resistance, the time constant of any such complete generator-motor circuit is the same as that of any other such complete generator-motor circuit. This equalization of the time constant of the several circuits would be present in the system as described, even where the bus connections at one side of the generators (for instance the side including buses 22, 23, 24, 25 and 30) were shorter or of diiferent resistance as compared with the bus connections at the other side of the generators (26, 27, 28, 29 and 31).

The equalization of the time constants of the circuits just referred to is of great importance during the rise and decay of the severe transients, since the time constant equalization results in substantially equal time-rate-ofchange of current in each circuit. Thereby the transient loads are equalized as between generators.

Notwithstanding the fact that the bus bars and other connections of the system are large physical elements, great accuracy is required with respect to the equalization of the resistance of the current paths in the manner referred to. indeed, establishment of current paths of equal resistance is actually complicated by the very fact that the bus connections must be of very substantial section and therefore of very low resistance, and the further fact that the installation as a whole is necessarily quite large, spreading over a length, for example, of as much as 50 to feet. This necessitates the construction of a bus system incorporating a multiplicity of bus bar members with joints therebetween, and it is impractical to construct such joints of identical resistance. The resistance of the joints in any one path represents a very large factor in the total resistance of any particular current path. Because of these factors not even a mechanically symmetrical bus system will necessarily provide current paths of equal resistance.

With the foregoing factors in mind the invention contemplates the employment of certain adjustable connectors between buses so that, by accurate measurement, the resistance value of any path may be matched with that of any other path. One desirable manner of accomplishing this adjustment is illustrated in Figure 1. Note for example that buses 24 and 25 are arranged parallel to each other and that they are interconnected by a cross connection 33 which is slidable along the parallel buses 24 and 25 so as to provide for interconnection of these buses at the desired point. Similar connectors are used between the intermediate bus 24 and the buses 22 and 23, and also in other places in the system, as is shown in Figure 3.

The slidable connectors 33 may also be adjusted in senses to provide generator-to-motor circuits having equal time constants in those situations where the cross connections between the series field windings are not of equal resistance, and even in situations where minor variations occur in the electrical characteristics of the generators or motors themselves.

It may be mentioned that in certain installations where it is found that the resistance of the several paths of the bus bar system at one side of the generators is substantially equal, the employment of slidable connectors at that side may not be necessary, in which event all of the compensation may be effected by appropriate adjustment of the slidable connectors in the bus bar system at the other side of the generators.

I have found that the employment of the features above described results in such accurate equalization of load on the generators at times of rapid current fluctuation that no one of the generators need carry more than its share of the load, even for an instant. In consequence substantial equalization of commutator action is established as between generators, since the action of the interpole and compensating windings, tending to neutralize armature reaction, is enhanced.

I claim:

1. A rolling mill comprising: a pair of rolls between which metallic blanks are adapted to be intermittently passed; a motor connected to drive one of said rolls, the motor having power terminals, entry of the blank into the rolls causing the motor to draw current at a high rate of rise; a plurality of generators to supply current for said motor, the generators having substantially equal electrical characteristics and each generator having power terminals; and current-carrying connections between the motor terminals and the terminals of the generators constructed and arranged to present a plurality of circuits wherein the time constants are substantially equal, each circuit including said motor and one of said generators.

2. A power system comprising: a load which is subjected to rapid and heavy fluctuations; a motor connected to said load, variations in the load causing the motor to draw current at high rates of rise, the motor having power terminals; a plurality of generators to supply current for said motor, the generators having substantially equal electrical characteristics and each generator having power terminals; and current-carrying connections between the motor terminals and the terminals of the generators constructed and arranged to present a plurality of circuits wherein the time constants are substantially 7 equal, each circuit including s'a'id motor and one of said generators.

3-. 'A rolling mill comprising: a pair of rolls between which metallic blanks are adapted to be intermittently passed; a motor connected to drive one of saidrolls, the motor having power terminals, entry or" the blank into the rolls causing the motor to draw current at a high rate of rise; a pair of generators to suppiy current to said motor, the generators having substantially equal electrical characteristics and each generator having power terminals; and current-carrying connections between the motor terminals and the terminals of the generators comprising positive and negative buses of substantially equal resistance value respectively interconnecting the positive terminals and the negative terminals of the generators, and a pair of motor feeding buses of substantialiy equal resistance value respectively interconnecting the motor terminals with the mid-poiuts of said positive and negative buses, said mid-points of connection being chosen to respectively divide each of the positive and negative buses into two portions of substantially equal resistance values whereby to provide two circuits whose time constants are substantially equal, each circuit including said motor and one of said generators.

4. A rolling mill comprising: a pair of rolls between which metallic blanks are adapted to be intermittently passed; a motor connected to drive one of said rolls, the motor having power terminals, entry of the blank into the rolls causing the motor to draw current at a high rate of rise; two pairs of generators to supply current to said motor, the generators having substantially equal electrical characteristics and each generator having power terminals; and current-carrying connections between the motor terminals and the terminals of the generators comprising first positive and negative buses of substantially equal resistance value respectively interconnecting the positive terminals and the negative terminals of one pair of generators, second positive and negative buses respectively interconnecting the positive terminals and the negative terminals of the second pair of generators, said second positive and negative buses being of substantially equal resistance value to each other and to eachof said first positive and negative buses; an intermediate positive bus connected to the mid-points of said first and second positive buses; an intermediate negative bus of substantially the same resistance value as said intermediate positive bus connected to the mid-points of said first and second negative buses; and positive and negative motor feeding buses of substantially equal resistance value respectively interconnecting said motor terminals with the mid-points of said positive and negative intermediate buses, said mid-points being chosen to res'pec'tively divide each of said positive and negative buses and each of said intermediate buses into two portions of substantially equal resistance value whereby to provide four circuits wherein the time constants are substantially equal, each such circuit including said motor and one of said generators.

5. A power system comprising: two loads jointly subjected to rapid and heavy fluctuations; two motors of substantially equal electrical characteristics each connected to a load, variations in the loads causing the motors to draw current at high rates of rise, the motors having power terminals; four generators to supply current for said motors, the generators having substantially equal electrical characteristics and said generators having power terminals; and current-carrying connections between the motor terminals and the terminals of the generators, comprising first positive and negative buses ofi substantially equal resistance value respectively interconnecting the positive and negative terminals of one pair of generators, second positive and negative buses respectivcly interconnecting the positive and negative terminals of the other pair of generators, said second positive and negative buses being of substantially equal resistance value to each other and to each of said first positive and negative buses, a positive intermediate bus connected to the mid-points of said first and second positive buses, a negative intermediate bus of substantially the same resistance value as said positive intermediate bus connected to the mid-points of saidfirst and second negative buses, positive and negative motor interconnecting buses of substantially equal resistance value respectively interconnecting the positive and negative terminals of said two motors, and positive and negative motor feeding buses of substantially equal resistance value respectively substantially equal, each circuit including one of saidmotors and one of said generators.

References Cited in the file of this patent UNiTED STATES PATENTS 1,590,854 Rothera et al June 29, 1926 1,760,278 Newlin et al May 27, 1930 1,916,443 Shirk July 4, 1933 1,953,792 Winne et al Apr. 3, 1934 2,576,117 Holmes Nov. 27, 1951 2,590,265 Miner et al Man 25, 1952 

1. A ROLLING MILL COMPRISING: A PAIR OF ROLLS BETWEEN WHICH METALLIC BLANKS ARE ADAPTED TO BE INTERMITTENTLY PASSED; A MOTOR CONNECTED TO DRIVE ONE OF SAID ROLLS, THE MOTOR HAVING POWER TERMINALS, ENTRY OF THE BLANK INTO THE ROLLS CAUSING THE MOTOR TO DRAW CURRENT AT A HIGH RATE OF RISE; A PLURALITY OF GENERATORS TO SUPPLY CURRENT FOR SAID MOTOR, THE GENERATORS HAVING SUBSTANTIALLY EQUAL ELECTRICAL CHARACTERISTICS AND EACH GENERATOR HAVING POWER TERMINALS; AND CURRENT-CARRYING CONNECTIONS BETWEEN THE MOTOR TERMINALS AND THE TERMINALS OF THE GENERATORS CONSTRUCTED AND ARRANGED TO PRESENT A PLURALITY OF CIRCUITS WHEREIN THE TIME CONSTANTS ARE SUBSTANTIALLY EQUAL, EACH CIRCUIT INCLUDING SAID MOTOR AND ONE OF SAID GENERATORS. 